1. Field of the Invention
The present invention relates to a linear illumination device for illuminating a surface of a document in an optical image reading apparatus such as a direct contact type image sensor unit.
2. Description of the Related Art
Optical image reading apparatuses have been widely used in apparatuses such as a compact facsimile machine or a bar cord reader which optically read a document. Such an optical image reading apparatus illuminates the document, receives the light reflected by the document, and then obtains electric signals corresponding to an image on the document in accordance with the amount of the reflected light. As an illumination device of the apparatus of this kind, an LED array constituted by LED chips arranged in a line is used.
With reference to the drawings, an example of the conventional linear illumination device used as the optical image reading apparatus will be described.
FIG. 28 shows the configuration of a conventional optical image reading apparatus. In FIG. 28, a document 141 is placed below the optical image reading apparatus. The optical image reading apparatus includes: an LED array as a linear illumination device 142 for illuminating the document 141; a rod lens array 143 for focusing light beams reflected by the document 141; and a photoelectric conversion element array 144 for receiving the focused light beams so as to convert the light beams into electric signals. As shown in FIG. 29, the LED array is constituted by arranging a plurality of LED chips 152 in a linear manner on a substrate 151 on which a circuit conductor layer is formed.
The operations of the optical image reading apparatus and the linear illumination device having the above configurations will be described below.
First, light beams emitted from the LED array 142 are radiated onto the document 141 to be read. The light beams reflected from the document 141 are focused by the rod lens array 143, and then are directed to the photo-electric conversion element array 144 so as to convert the light beams into electric signals corresponding to an image on the document 141.
In general, the document 141 is optically read while the optical image reading apparatus is scanning the document 141. In the case of using the LED array 142 as the illumination device, a direction along which the document 141 is scanned (hereinafter, simply referred to as a sub-scanning direction) is perpendicular to a direction in which the LED chips are arranged. In order to accurately read the document 141, the optical image reading apparatus requires that the illumination device illuminates a portion of the document 141 with a narrow width in the sub-scanning direction. In addition, illumination is required to be uniform in a direction perpendicular to the sub-scanning direction (hereinafter, referred to as a main scanning direction).
In the case of using the LED array 142, however, it is difficult to illuminate the document 141 uniformly in the main scanning direction due to variation in the amount of light emitted from each of the LED chips 152 and effects of the directionality thereof. In order to reduce the adverse effect of the directionality of the LED chips 152, the number of the LED chips 152 needs to be increased. Alternatively, when the distance between the surface of the document 141 and the LED array 142 is made larger, the effects of the directionality of the LED chips 152 can be reduced. For example, in the case where an array of 24 LED chips is used as the illumination device, the distance between the document and the LED array should be set to be 9-10 mm in order to illuminate an A4 sized document with a satisfactory uniform light.
If illumination is not uniform in the main scanning direction, the electric signals obtained in accordance with the amount of light received by the photoelectric conversion element array 144 are also poor in uniformity (PRNU). The poor uniformity of the electric signals increases the production cost of the optical image reading apparatus in the case where the obtained electric signals are subjected to a signal correction processing (for example, shading correction). In addition, the electric signals with poor uniformity burden the signal correction processing ability. On the other hand, in the case where the signal correction processing is not performed, for example, when a uniformly gray document is read by the optical image reading apparatus, a brightly illuminated part may be displayed as white. Likewise, an insufficiently illuminated dark part may be displayed as black.
FIGS. 31A and 31B show cross-sectional views of a direct contact type image sensor unit using the above-mentioned conventional illumination device. A document 64 is placed so as to be in close contact with one end of an optical fiber array 63 and is irradiated with light from an LED array 65 placed above. The reflected light which carries information of the document is directed toward a light receiving array 62 which is provided on the other end of the optical fiber array 63 so as to be converted into image signals.
In the image sensor unit as described above, however, illuminance on the surface of the document greatly varies since the LED array 65 is used as the illumination device. Therefore, since sensitivity of the sensor varies greatly, image reading performance is deteriorated. Moreover, since it is necessary to space the document 64 from the LED array 65 as described above, the unit itself becomes large. Therefore, a larger number of LED chips are required, thereby raising the cost of the unit.
Moreover, when the LED array 65 is brought closer to the surface of the document 64 in order to increase an S/N ratio, PRNU of the electric signals are further deteriorated due to the adverse effect of the directionality of each of the LED chips.
Next, another example of a conventional optical image reading apparatus will be described with reference to FIG. 30.
FIG. 30 shows the configuration of another conventional optical color image reading apparatus. In FIG. 30, three fluorescent lamps 142R, 142G and 142B are used as an illumination device. The three fluorescent lamps 142R, 142G and 142B are respectively for red light, green light and blue light (hereinafter, respectively referred to simply as R, G and B). The fluorescent lamps 142R, 142G and 142B are each lit separately in a time divided manner. A colored light beam emitted from one of the respective fluorescent lamps is reflected by a document 141 so as to be focused onto a photoelectric conversion element array 144 by a rod lens array 143. The photoelectric conversion element array 144 receives the focused light beam to convert it into an electric signal. The operation is successively repeated for R, G and B, thereby allowing the color of the document 141 to be analyzed.
In this configuration, the document 141 can be illuminated uniformly in the main scanning direction. However, the three fluorescent lamps 142R, 142G and 142B respectively corresponding to R, G and B are required, making it difficult to realize low cost and reduction in size of the optical color image reading apparatus.